[RFC] Use `dune shell` for testing failing command

When an action fails we currently print a little sh script which should run the command in the same way:

(cd _build/default/foo/bar && ../my.exe arg1 arg2 > file1 )

However we have to go to great length to translate some action features such as redirection, pipe, that we are not always sure to translate correctly. Moreover dune adds also a lot of environment variables such as OCAML_LOADPATH, and also dune specific one for the sites and plugin mechanism. So running executable directly from _build is a bad habit and will fail in not simple cases.

dune exec execute correctly executable with the needed environment. We can perhaps make it able to execute actions so that the output of failed command is simpler to write and more truthful but still easy to modify from the command line:

dune exec --in-build-dir --chdir foo/bar --context default --with-stdout-to file1 -- ../my.exe arg1 arg2

Start by tracing how failed actions are rendered into shell scripts and how dune exec handles --in-build-dir, --chdir, --context, and output redirection. Compare the current failure command with the proposed dune exec form; done means failed commands can be reproduced with the needed Dune environment without manually translating action features.